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Rounded cobbles that have not travelled far: incorporation of corestones from saprolites in the South Mountain area of southern Nova Scotia, Canada



Saprolitic palaeosurfaces occur at several localities on the granitoid rocks of the South Mountain Batholith of Nova Scotia. There are three ages of saprolites within the study area: pre-Pleistocene, pre-Triassic and pre-Carboniferous. Within these ‘in-place’ weathered horizons, there are remnant ellipsoidal blocks of unweathered granitoid referred to as corestones. These corestones are isolated rounded pods of relatively unweathered material surrounded by rotted granitoid saprolitic material. The weathered material which surrounds these corestones is poorly consolidated and easily eroded. The erosion of these horizons produces a lag deposit that contains many rounded corestones which can be incorporated into subsequent sedimentary units. The rounded boulders, cobbles and pebbles of granite within many of the Pleistocene glacial deposits in southern Nova Scotia are probably related to the incorporation of these saprolite related structures, given the locally derived (within 400 m of the source) nature of the tills. The presence of saprolites at unconformities of various ages on the South Mountain granitoid rocks suggests that incorporation of saprolitic material probably occurred along a number of palaeosurfaces in the past. The recognition of this process has implications for the interpretation of rounded granite-clast conglomerates and quartz-rich sandstones of various ages within the stratigraphic record of eastern Canada. Similar palaeosurfaces elsewhere in the world also have related saprolite derived sedimentary rocks associated with them. In summary, well-rounded spherical pebbles, cobbles and boulders of granitoid material incorporated in sedimentary strata need not have travelled far from source nor are they necessarily recycled from older conglomerates.